1. Field of the Invention
This invention relates generally to positron emission tomography and more particularly to devices which use an array of scintillation detectors to detect the annihilation radiation from positron disintegration and use this information to reconstruct an image of the distribution of positron emitting isotopes within a body.
2. Description of the Prior Art
Positron emission tomography is a technique for measuring the concentration of a positron emitting isotope through a sectional plane through the body. Normally the isotope is used to label a substance which circulates with the blood and may be absorbed in certain tissues. The technique allows the actual concentration in the slice to be determined if the device is suitably calibrated.
Certain isotopes decay by emitting a positively charged particle with the same mass as the electron (positron) and the neutrino from the nucleus. In this process one of the protons in the nucleus becomes a neutron, so that its atomic number goes down while its atomic weight remains constant. This positron is ejected with a kinetic energy of up to 2 MeV depending on the isotope and loses this energy and collisions while travelling a distance of up to a few mms in water. When it has reached thermal energies it interacts with an electron and they mutually anhilate one another. The rest mass of the 2 particles is transformed into 2 gamma rays of 511 keV which are emitted at 180.degree. in the `center of mass` coordinates of the original particles. The 2 gamma rays may be detected by suitable devices. If these devices measured the energy of the gamma rays at 511 keV and register this energy almost simultaneously it may be assumed that the original of the radiation is on a straight line between the 2 detectors. Several detectors may be used in an arrangement so that many coincident events may be imaged during the same time interval. Then the information from these detectors is processed by a computer using image reconstruction techniques in order to find the location of distribution of positron emitting isotope.